This invention relates to ink jet printing, and more particularly to ink jet ink containment and provision of ink level determination.
In ink jet printers, ink jet print cartridges or xe2x80x9cpensxe2x80x9d are reciprocated on a carriage to print swaths on an advancing media sheet. Pens typically include an ink chamber filled primarily with ink, with a print head having an array of nozzles for expelling ink droplets in a controlled pattern. Many existing pens are self contained units that are discarded when ink is depleted, however, the ink chamber can be constructed as an independent container that is removable from the print head. As ink is consumed, it is desirable for the user to know the amount of ink remaining in the cartridge. This allows new pens to be purchased adequately in advance, and permits replacement of nearly depleted pens or ink containers to be made before initiating large print jobs, or before allowing an ink jet facsimile machine to be unattended for an extended period.
Existing ink jet printing systems employ various approaches to provide ink level information to the user or to the printer control circuitry. One technique employs a xe2x80x9cdrop countingxe2x80x9d circuitry that counts the number of ink droplets printed, and uses this to estimate the total quantity of ink consumed. While this is an adequate estimate for some purposes, it does not provide a reliably absolute measurement, since drop volumes are generally not precisely controlled. In addition, the technique requires electronic resources to be dedicated to the ongoing calculation, increasing printer and pen cost and complexity.
Other techniques have sought to estimate pen ink levels with electronic sensing of the thermal capacitance of the ink mass by heating a semiconductor chip adjacent the ink, and measuring the rate of conduction or cool-down, which varies with the ink quantity. This is a complex and inaccurate technique, as is the technique of measuring electrical resistance across the ink mass.
Visual fluid gauges have long been employed in simple fluid containers having a xe2x80x9cwindowxe2x80x9d for viewing the fluid level in an otherwise opaque container. However, most ink jet devices contain pressure regulation systems in the container which make it difficult to see the ink and establish an ink level. More particularly, in ink jet pens employing capillary foam for retaining the ink with a negative head to avoid ink xe2x80x9cdroolxe2x80x9d from the nozzles, the ink-carrying foam does not provide visual evidence of its saturation level. An xe2x80x9cemptyxe2x80x9d pen will appear much the same as an xe2x80x9cfullxe2x80x9d pen, because the foam in the empty pen contains significant residual ink. And even if there were a slight appearance change between xe2x80x9cfullxe2x80x9d and xe2x80x9cempty,xe2x80x9d this would not be reflected in a discernible xe2x80x9clevelxe2x80x9d that indicated at any given moment the pen fill status; the entire foam would appear substantially uniform under all circumstances.
The present invention overcomes the limitations of the prior art by providing an ink jet print cartridge with a body having a first ink chamber. The body also has a second ink chamber, with a fluid passage connecting the chambers. The second ink chamber has an elongated capillary portion with a light transmissive window revealing the presence of ink in the capillary portion. The first ink chamber may be the primary ink storage chamber, and may contain an ink-retaining foam element that contacts a capillary feature of the passage. The foam may be compressed at one portion to provide a capillarity gradient, and the second chamber capillary may also have a capillary gradient.